Crosstalk between lipid metabolism and mitochondrial bioenergetics in tuberous sclerosis complex
Kavanagh, Taylor Rose
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BACKGROUND: Tuberous sclerosis complex (TSC) is a multisystem hamartomatous disease caused by inactivating mutations in the TSC1/TSC2 genes leading to hyperactivation of mammalian target of rapamycin complex 1 (mTORC1) in TSC tumors. Novel therapeutic regimens and imaging biomarkers remain critical unmet needs in TSC. Mitochondrial fatty acid oxidation (FAO) is a major cellular source of acetyl-CoA. Acetyl-CoA is a central metabolite in lipid anabolism and catabolism. The purpose of this study was to identify novel metabolic therapeutic targets, particularly involving lipid metabolism, to achieve durable responses in TSC. METHODS AND RESULTS: Fluoroacetate (FACE), an acetate derivative, is a surrogate biomarker of mitochondrial activity. It accumulates in the mitochondria without being oxidized to CO2, and it is converted to fluorocitrate, irreversibly inhibiting the TCA (tricarboxylic acid) cycle enzyme aconitase. Micro-positron emission tomography (PET) imaging of subcutaneous xenografts of TSC2-deficient Eker rat uterine leiomyoma-derived ELT3 cells showed rapid uptake of [18F]FACE that was maintained after 72-hour treatment with mTORC1 inhibitor rapamycin. This result suggests that mitochondrial oxidative metabolism is sustained in the presence of rapamycin. Consistent with this finding, treatment of TSC2-deficient cells with rapamycin led to a significant increase in FAO and a decrease in glucose oxidation in vitro as measured by a 14C-CO2 collection metabolic assay. Expression of the A isoform of carnitine palmitoyltransferase I (CPT1A; FAO rate-limiting enzyme) was selectively increased in TSC2-deficient cells. Pharmacological inhibition of CPT1A by ST1326 led to a decrease in FAO, as measured by a 14C-CO2 collection metabolic assay, and a decrease in mitochondrial ATP production, measured by the Seahorse analyzer. CONCLUSIONS: This study proposes a role for FAO in TSC tumor bioenergetics and for CPT1A as a potential therapeutic target in TSC.